In typical well cementing operations, a cement slurry is prepared at the surface and pumped into the well through a liner or casing to fill the annulus between the casing and borehole wall to provide zonal isolation and mechanical support. The cement slurry should preferably present relatively low viscosity and have effectively constant rheological properties while it is being prepared and pumped into the well and placed in the zone that is to be cemented. Once it is in place, the cement will ideally develop high compressive strength in a minimum of time. The time to develop the compressive strength is a function of the temperature but will also depend strongly on the water to cement ratio. It is well known that extended slurries (i.e. slurries having a high water content, typically to achieve reduced density) can take a long time to develop sufficient compressive strength and contribute to increase the rig time taken up in the cementing operation.
Cement slurries in widespread use for oil and gas wells typically have a volume fraction of water (volume of water/total volume of slurry, sometimes called ‘slurry porosity’) of about 59%, which corresponds to a water to cement weight ratio of about 44 wt %. It is generally accepted that only about 22 wt % of water is needed for the hydration of the Portland cement, the excess water in the slurry causing the development of porosity in the set material. While a water to cement ratio of 44 wt % can lead to a set material having a sufficiently high compressive strength and an acceptable permeability, the same is not true in the case when lighter slurry densities are required.
Lightweight cement slurries are typically designed using one of three technologies: extended slurries, foam cements and engineered particle size systems.
In extended slurries, the slurry density is decreased by increasing the water to cement ratio, typically up to 100 wt % to achieve a slurry density of 12.5 ppg (1503 kg/m3). With such a high amount of water, the development of the compressive strength is slow and the set material exhibits a high permeability and a low compressive strength (less than 1000 psi (0.69 MPa)).
In foam cements, a base slurry having a typical water to cement ratio of 44 wt % is foamed with a gas (usually nitrogen). The water to cement ratio is kept constant when adding the gas. In this case, the rate of development of the compressive strength is not affected compared to the base slurry, but the gas introduced in the material generates porosity leading to a significant increase of the permeability and decrease of the final compressive strength (typically 2200 psi (1.52 MPa) for a 12.5 ppg (1503 kg/m3) slurry).
In engineered particle size systems, such as those described in EP 0621247 A (SOFITECH NV) 26.10.1994 and WO 0109056 A (SOFITECH NV) 08.02.2001, the cement is blended with other particles so that the packing volume fraction of the solids is optimized, which allows reduction of the amount of water needed to maintain good rheological properties. This technology is an improvement compared to the previous ones, as the porosity of the set material remains low whatever the slurry density (which can be controlled by selecting particulate materials of suitable density to form the slurry) and high compressive strength can be achieved even if the water to cement ratio in such slurries is generally not below 50 wt %.
It is an object of the invention to provide a cement slurry system that can be prepared with low water to cement ratios while rapidly developing a high compressive strength.